This invention relates to optical scanning of a stationary surface with a rotating mirror. A typical environment where the invention might be used is in a photocopying apparatus in which an original object lies on a flat platen for scanning and image projection by a rotating mirror.
There are three general types of strip scanning arrangements known to the prior art. In one, the optical system is fixed and the object and photoreceptor move relative to the optics. In the second, the object is fixed and the optical system and photoreceptor move at appropriate speeds relative to it. In the third, the object is fixed and is scanned by a rotating mirror to relay an image of the object onto a moving photoreceptor. It is this third type system to which the present invention is directed.
Typically, in the prior art, rotating mirror systems have required a curved platen on which to support the object to be copied. The radius of curvature of the platen is equal to the platen-mirror axis distance in order to maintain constant the object conjugate distance as has generally been required by the optics.
It is desired for reasons of economy and utility to provide a mirror scanning system in which a flat platen may be used instead of the prior art curved platen. The problems inherent in scanning a flat platen with a rotating mirror include the optical problem of maintaining a constant object conjugate distance while scanning and the mechanical problem of scanning the object platen at a constant speed so as to "lay down" an image in synchronism on a photoreceptor which is moving at a constant surface speed.
Accordingly, it is an object of this invention to provide a flat platen, rotating mirror scanning system in which the object plane is scanned by the rotating mirror at a constant scan velocity.
Another object is to provide such a system in which object and image conjugate distances are held in constant proportion, e.g. 1:1.
Briefly, this invention is practiced by a flat platen, rotating mirror scanning system in which the angular velocity of the rotating mirror is continually varied from end to middle to end of scan to effect a constant speed of scan across the platen surface. This is achieved by providing a mirror drive arrangement which is the mechanical analog of the optical geometry. Conjugate distance changes inherent in flat platen scanning are compensated by a stationary "half-lens" adjacent to the rotating mirror.